OP, good for you wanting to change things up.
I've really enjoyed this thread because a friend of mine asked me for advice on this recently. She'd just read a book called The Start Up of You (or something) by the Linked-In guy. She was really direct about wanting to change herself and get her family moving in more cultured circles. I have complete respect for that kind of aspiration. Why not go for something you like the look of?
Lots of my mates dress in 'County Set' style. It's a pretty good choice for a new look as it really can be done at any price point. And you won't look so fashionable that it will need updating quickly.
Fortunately, unlike aspiring to be a WAG or fashionista, posh people don't give a stuff about bags (usually left in the car) so that's one expensive item you don't have to invest in.
Tweed is never too far away (this is good for a handbag, jacket also).
Shoes should never be very high.
No tan, except a bit of colour from skiing/riding.
Dubarry/Le Chameau if you have money. Any other wellies/long brown leather boots if you don't. When I say any other wellies, obviously, these must be plain coloured. No fun boots.
Fun shirts are fine though, worn under v-necks.
Lambswool jerseys.
Posh people don't really know about nail art. Don't do it.
Most posh people aren't rich, or at least, don't consider themselves wealthy so dislike displays of wealth. No logos except unavoidable Joules/Boden type.
Shop in John Lewis - always, for everything.
DO: cook from scratch, go to point-to-points, make friends with mums at dc swimming, read books, suggest going to exhibitions you might like, go on nature walk-type organised activities at National Trust places.
DON'T: ever talk about cleaning or keeping your house clean or wanting the house to be clean or remarking on how clean somebody's house is (dead giveaway). Shoes by the door, you can do this, but it will be noted. Don't be allergic to dogs or frightened of them (seriously).